As a track spring that elastically supports an idler and exhibits a cushioning function in a crawler undercarriage attached to an excavator or the like, there is a track spring in which the spring pitch is made different in some coil sections of the entire coil spring with respect to other coil sections so as to form bite preventing sections that can prevent the biting of earth, sand, and the like (JP-A No. 2006-044561).
The above-described track spring is also an open end coil spring that has gaps between its end sections and its first coil sections in a free state.